A temperature tracking circuit for driving a laser diode incorporating an inner loop which includes a light level detector for regulating the laser diode forward voltage, and a voltage-to-current converter for driving the laser.
A laser diode is typically set to its correct operating levels outside of the active scan region, but during active scan, the temperature of the laser can vary depending on the momentary duty cycles. For example, after a long string of "ON" or "OFF" pixels, the instantaneous temperature of the diode may rise or fall, and the output light intensity (power) may vary.
If the laser is being driven by a voltage source, the laser is self-correcting to some extent. That is, the variation in laser temperature is accompanied by a change in output power, as well as a change in forward voltage drop in laser diodes these two parameters track well. However, the problem with a voltage source is that it is sensitive to the capacitative elements of the laser, and is not a driver of choice in high data rate systems.
A current driver is the driver of choice that enables high data rates; however, the current drive and the laser output power do not track as well as voltage to power. An optimum design will have to incorporate the high bit rate capability of the current drive while retaining the inherent advantage of voltage and power tracking with temperature.